Mitt Romney joined a growing chorus of criticism over a pro-Barack Obama ad linking the Republican candidate's time at Bain Capital to the death of a woman from cancer.

Conservatives expressed disgust at the ad, describing it as despicable and a new low in the campaign. Romney's former Republican rival, Newt Gingrich, said it was "truly tragic" that such an ad should be running in a presidential campaign.

One of the main Super Pacs backing Obama re-election efforts, Priorities USA Action, released the ad on Tuesday evening. But in spite of the heavy criticism, the group is refusing to back down and will go ahead with plans to run it in swing states.

In the ad, former steelworker Joe Soptic recounts how when Bain Capital, which Romney headed, shut down his steel plant in 2001, he lost his family health benefits. His wife died of cancer five years later.

"I do not think Mitt Romney realises what he's done to anyone. And furthermore, I do not think Mitt Romney is concerned," Soptic says in the ad.

The Romney campaign said the ad is misleading: the woman died years after Romney left Bain Capital – running the Salt Lake Olympics rather than Bain in 2001 – and that the woman had health coverage through her own job.

The Republican campaign has been critical of the ad since it was first made public but Romney only commented for the first time on Thursday.

In an interview, Romney told Bill Bennett's Morning in America radio show: "I don't know what happened to the campaign of hope and change."

Without referring directly to the ad, he said: "You know, in the past, when people pointed out that something was inaccurate, why, campaigns pulled the ad. They were embarrassed. Today, they just blast ahead. You know, the various fact-checkers look at some of these charges in the Obama ads and they say that they're wrong, and inaccurate, and yet he just keeps on running them."

The row over the ad comes as polls show the White House race almost tied. Real Clear Politics, in its poll average, puts Obama on 47.5% to Romney's 44.1%.

But Romney is set to dominate headlines in the coming weeks, firstly with his vice-presidential pick and then with the Republican convention, beginning in Tampa, Florida, on 27 August.

After being formally nominated as the Republican presidential candidate, he will be free to spend the millions of dollars in campaign funds he has been accumulating in recent months.

Bill Burton, founder of Priorities USA Action, in an interview on Wednesday night insisted the ad will be run as planned in swing states at a cost of about $20m. He denied the ad suggests Romney was responsible Soptic's wife's death

"We're not saying Mitt Romney is culpable for that. What we're saying is that Joe Soptic was fired from his job, and as a result of that, he wasn't able to get … he wasn't able to hold onto healthcare benefits that were promised to him. And as a result, when his wife got sick, he didn't have healthcare."

Another of Romney's former Republican rivals, Tim Pawlenty, also joined the criticism. In comments distributed by the Romney campaign, Pawlenty accused Obama supporters of putting out negative ads to distract attention from his record as president.

Pawlenty, former governor of Minnesota, is being talked of as a possible candidate to becoming Romney's vice-presidential running-mate. Other potential picks include congressman Paul Ryan and Ohio senator Rob Portman. The Romney campaign said the decision will be announced "soon".